1. Field of the Invention
The present invention relates to the preparation of nonwoven fibrous sheet materials containing filler materials and a process for making said sheet.
2. Description of the Related Art
Plexifilamentary sheet material containing fillers is known. U.S. Pat. No. 3,081,519 (Blades et al.) and U.S. Pat. No. 3,169,899 (Steuber) disclose the addition of common textile additives such as dyes, pigments, antioxidants, delusterants, antistatic agents, reinforcing particles, removable particles, and U.V. stabilizers to the polymer used in a process for forming fibrillated strand materials. U.S. Pat. No. 5,512,357 (Shimura et al.) discloses a process for making a plexifilamentary fiber involving adding 0.1 wt % to 11 wt % of a spreading agent to the polymer. The spreading agent may be a nucleating agent, a lubricant or a crystalline resin except a base resin. U.S. Pat. No. 6,010,970 (McGinty et al.) discloses a sheet material flash spun from polyolefin and a pigment wherein the pigment comprises between 0.05 wt % and 10 wt % of the flash spun fibril strands. The pigment is added to increase the opacity of the flash spun sheet.
The art of flash-spinning plexifilamentary film-fibrils from a polymer in a solution or a dispersion is known in the art. The term “plexifilamentary” means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean thickness of less than about 4 micrometers and with a median fibril width of less than about 25 micrometers. In plexifilamentary structures, the film-fibril elements are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form the three-dimensional network.
The process of forming plexifilamentary film-fibril strands and forming the same into non-woven sheet material has been disclosed and extensively discussed in U.S. Pat. No. 3,081,519 to Blades et al.; U.S. Pat. No. 3,227,794 to Anderson et al.; U.S. Pat. No. 3,169,899 to Steuber; U.S. Pat. No. 3,860,369 to Brethauer et al.; and U.S. Pat. No. 5,603,885 to McGinty (all of which are assigned to DuPont). This process and various improvements thereof have been practiced by DuPont for a number of years in the manufacture of its TYVEK® spunbonded olefin.
The polymers that have been conventionally used in production of flash-spun plexifilamentary sheets are polyolefins, especially polyethylene. The term “polyethylene” is intended to embrace not only homopolymers of ethylene but also copolymers wherein at least 85% of the recurring units are ethylene units. A preferred polyethylene polymer is a homopolymeric linear polyethylene, which has an upper limit of melting range of about 130° to 135° C., a density in the range of 0.94 to 0.98 g/cm3 and a melt index (as defined by ASTM D-1238-57T, Condition E) of 0.1 to 6.0. Polypropylene is another polyolefin that can be used to make sheet material for use in packaging applications requiring higher temperature sterilization processes such as steam sterilization.
Unfortunately, it is difficult to maintain good sheet breathability in a spunbonded sheet with high liquid barrier and good physical properties. Known processes for effecting higher breathability also result in lower liquid barrier. Some end uses in protective apparel, such as medical fabrics, require a combination of good breathability and high liquid barrier. It is important for the material used in a medical gown to breathe to provide comfort for the wearer, however, it is also important for the material to resist the flow of fluids through the medical gown to the wearer.
Accordingly, there is a need for a sheet material having improved breathability without undergoing a significant reduction in the physical properties and/or the liquid barrier of the sheet.